1. Field of the Invention
This invention relates generally to a rail fastener and more particularly to a fastener for holding a rail onto a support structure which provides improved electrical isolation and vibration and sound attenuation between the rail and the support structure and permits improved lateral adjustment of the rail with respect to the support structure, while maintaining structural integrity between the rail and the support structure.
2. Prior Art
Direct fixation rail fasteners have been employed extensively in recent years in place of tie-on ballast arrangements for affixing transit rail apparatus to a rigid support structure. Because of the stress conditions placed on the rail and supporting by the transit apparatus, as well as by changing environmental conditions, such as temperature, moisture, etc., direct fixation of a rail to a concrete support structure is not a simple matter. Structural integrity must be maintained between the rail and the support structure, but vibrations, including sound vibrations, which are generated in the rail must be attenuated before reaching the support structure. Direct fixation design is still further complicated by the fact that many of the transit systems are electrically energized and use the rail as the return path for the energizing electrical current, and as a result, the rails must be electrically isolated from the support structure. Also, such fasteners must be capable of permitting lateral adjustment or positioning of the rail with respect to the support structure. The most severe compromise, however, is that which must be achieved between attaining a desired amount of structural integrity between the rail and the support structure while sufficiently attenuating any vibrations which may be transmitted from the rail to the support structure.
As a rail mounted vehicle moves along a track, a differential wave is caused to build up in the rail in front of the vehicle because of the leverage action which results from the localized vertical forces applies to the rail by the wheels of the vehicle. Thus, a given portion of the rail is subjected to first an upward force as the vehicle approaches and then a downward force as the wheels roll thereover. Where the rail is directly affixed to the support structure, this wavelike motion will produce a pounding action between the rail and the supporting concrete structure which will tend to desintegrate the concrete unless some means is provided between the rail and the concrete structure to absorb the impact therebetween.
In addition to the deleterious effects on the concrete structure produced by the pounding action, undesirable sonic vibrations will be introduced to the surrounding structures. Thus, suitable means must be incorporated into the rail fastener device to absorb shock and dissipate some of the energy in order to attenuate the noise which would otherwise be transmitted into surrounding buildings and other structures.
Another problem which must be overcome in attaching a rail directly to a concrete support structure is that of maintaining gage accuracy between the rails. This is especially true in areas where the supporting structures will be subjected to sinking, earthquakes, and other uncontrollable phenomenon. Thus, means must be provided in direct fixation rail fasteners which will permit the rails to be adjusted laterally within reasonably limits. As an example, one current set of design specifications require that lateral adjustment be at least plus or minus one-eighth inch.
In addition to providing vibration attenuation and rail position capability, a rail fastener must also provide structural integrity between the rail and the support structure. However, a compromise exists between structural integrity and vibration attenuation, since structural integrity implies a relatively rigid fixation device between the rail and the support structure, while vibration attenuation implies a non-rigid fixation device. That is, a rail fastener must be sufficiently rigid to provide structural integrity between the rail on the support structure, but must be sufficiently non-rigid to be able to attenuate vibrations transmitted from the rail to the support structure. This problem is further compounded by the requirement that the fastener must be capable of permitting lateral adjustment or positioning of the rail with respect to the support structure. Such lateral positioning capability is incompatible with the requirements for structural integrity.
When a vehicle moves over a rail, in addition to the differential pressure wave discussed above, the rail will be subjected to overturning moments and shear forces, particularly in a curved portion of the track. If a rail is permitted to move laterally when lateral shear forces are imposed thereon, the gage of the track will not be maintained and the vehicle may lose contact with the rail. However, all of the known direct fixation rail fasteners which are capable of absorbing the above mentioned vertical forces do not achieve a proper balance between lateral restraint of the rail and vibration attenuation. That is, those prior known direct fixation rail fasteners which provide a sufficient amount of structural integrity between the rail and the support structure are not capable of sufficiently attenuating vibrations transmitted from the rail to the support structure. On the other hand, those direct fixation rail fasteners which are capable of sufficiently attenuating vibrations are not capable of providing a sufficient amount of lateral restraint and, therefore, structural integrity between the rail and the support structure.
In addition to the above mentioned problems encountered in the direct fixation of a rail to a support structure, prior known direct fixation rail fasteners have other disadvantages. Presently, the most widely used type of rail fastener employs a shear pad in which a layer of elastomeric material is sandwiched between two plates, with the rail being clamped to the top plate and the bottom plate being clamped to the support structure. These shear pads type of rail fasteners include structures for laterally restraining the top plate with respect to the bottom plate. Also, the majority of these rail fasteners are capable of positioning the rail laterally with respect to the support structure, but are not capable of adjusting the lateral position of the rail with respect to the support structure. Examples of such rail fasteners are disclosed in U.S. Pat. Nos. 3,576,293; 3,784,097; and 3,858,804.
The rail fasteners disclosed in these patents include a shear pad which is formed of a pair of metallic plates having a layer of elastomeric material sandwiched therebetween. The shear pad is secured to the support structure by a pair of studs and additional means are provided for laterally positioning the rail with respect to the shear pad and support structure. The lateral positioning structures disclosed in those patents include serrated members which are relatively difficult and costly to manufacture. Furthermore, this type of lateral positioning structure cannot be manipulated to laterally adjust the rail to a desired location on the shear pad. That is, these lateral positioning structures are not capable of moving the rail with respect to the shear pad and, therefore, the rail must be moved by additional means while the lateral positioning structures are being relocated. Accordingly, it can be appreciated that the lateral positioning means disclosed in the above-mentioned patents do not, in fact, adjust the lateral position of a rail, but hold the rail in a desired location after it has been positioned laterally with respect to the shear pad.
One of the problems encountered in the shear pad type of rail fastener is that of providing a sufficient amount of vibrational dampening while maintaining a desired amount of lateral restraint. The device disclosed in U.S. Pat. No. 3,576,293, laterally restrains the elastomeric layer by providing the bottom plate of the shear pad with an upturned flange for holding the lateral edges of the elastomeric layer. It was found, however, that with the incorporation of voids in the elastomeric layer to increase the vibrational dampening effect thereof, such an upturned flange did not provide the desired amount of lateral restraint to the elastomeric layer. Furthermore, lateral shear forces imposed on this upturned flange would eventually result in fracture thereof, thereby further decreasing the lateral restraint of the fastener. This problem was solved, as disclosed in U.S. Pat. No. 3,784,097, by the use of a nylon insert mounted between each anchor bolt and an edge of the upper plate of the shear pad. Any attempted lateral movement of the upper plate of the shear pad would bear against the nylon insert and impose a shear force on the anchor bolt or the sleeve surrounding it. It has been found, however, that this arrangement is unsatisfactory for a number of reasons.
Whenever attempted lateral movement of a rail imposes shear forces on a bolt or other anchor structure, such shear forces will eventually fatigue the anchoring fastener, ultimately resulting in failure thereof. In addition, such an arrangement does not provide a sufficient amount of vibration and sound attenuation between the rail and the support structure. Such a nylon insert, or any other noncompliant insert, transmits noise and other vibrations with relatively little attenuation. As previously mentioned, one of the requirements of such rail fasteners is to attenuate such noise to an acceptable level so that such noise will not be transmitted into the surrounding ground and to adjacent building.
Furthermore, the anchoring bolts of a fastener usually place the concrete which is in immediate contact therewith in tension when they are tightened to hold the fastener onto the concrete support structure. That is, these anchoring bolts are pulling the fastener and the concrete support structure together, thereby placing a portion of the concrete structure in tension. Any vibrations transmitted through the anchoring bolts to the concrete add transient forces to the pretensioned concrete. Such tensioning of the concrete around the anchoring bolts or the inserts to which they are threaded contributes to its ultimate fatigue. Pulverization of the concrete support structure in which the anchoring bolts are attached will eventually weaken that attachment. As that attachment weakens, the anchoring bolts will have greater freedom of movement, thereby further increasing the pulverization of the concrete support structure. Such movement of the anchoring bolt will also lead to fatigue thereof, with the end result being that either the anchoring bolt will fracture or the support structure will eventually lose its grip thereon.
In an attempt to overcome this problem, prior known rail fasteners employ the technique of clamping the bottom plate of the shear pad as tightly as possible to the surface of the support structure so that relatively little or no movement will exist when extreme lateral shear loads imposed thereon. However, this clamping of the bottom plate of the shear pad to the support structure does not eliminate the transmission of vibrations therethrough. Furthermore, tightly clamping the bottom plate of the shear pad to the supporting structure further increases the tension produced in that portion of the concrete support which grips either the anchoring bolt or the insert in which it is threaded.
In a further attempt to overcome this problem and in addition to clamping the bottom plate of the shear pad to the support structure, additional means have been provided for compressing the elastomeric layer, such clamping of the elastomeric layer reduces its ability to attenuate sound and other vibrations, with the result that such vibrations will be transmitted to the anchoring fastener and the support structure.
Others have attempted to solve the problem of attenuating vibrations produced by vertically directed forces by placing a layer of elastomeric material such as rubber, directly between a rail plate and the concrete support structure. However, all of these attempts have a direct connection between the rail plate and the support structure which provides structural integrity between the rail and the support structure, but does not attenuate any vibrations which are transmitted from the rail, through the rail plate and the anchoring devices to the support structure. These devices are not, in fact, shear pads, since they do not permit even a limited amount of lateral movement of the rail plate with respect to the support structure. In the absence of such lateral movement, and because of the direct connection between the rail plate and the support structure, vibrations are not attenuated. In effect, this type of rail fastener is only capable of dampening those vibrations which are the result of vertical forces applied to the rails by the wheels of the vehicle passing thereover. All of the prior known fasteners of this type have employed an elastomeric material such as rubber which is highly abrasive. As a result, this type of rail fastener has not proven satisfactory in use over a prolonged period of time because of the ultimate destruction of the elastomeric layer. An example of such a fastener is disclosed in U.S. Pat. No. 2,146,341.
The shear pad type of rail fastener is also subject to a loss of structural integrity between the rail and the support structure due to failure of one or more parts thereof. In the shear pad type of rail fastener, it has been the practice to provide voids in that portion of the elastomeric layer which is directly beneath the rail, such that its dampening effect on vibrations will be increased. The portions of the elastomeric material, however, which extend to the edges of the rail plate are not so relieved. As a result, whenever a load is placed on the rail, the rail plate will bow, since the edges thereof are held from downward movement by the solid elastomeric material, whereas the center portion thereof which is beneath the rail is permitted to move vertically. Continuous flexure of the rail plate will eventually result in its becoming fatigued. Many of the prior known fasteners of the shear pad type provide slots or other openings in the rail plate for receiving other members therein, such as clamping bolts. The absence of material in these areas further increases the likelyhood of structural failure of the rail plate under such flexural conditions.
It has also been the practice in the past to bond the elastomeric material to the surface of any elements which join the top and bottom plates of the shear pad. The elastomeric material at those areas will eventually fail under prolonged and repeated flexure of the rail plate. Such failure of the elastomeric material at those areas also reduces the structural integrity of the rail fastener.
As previously mentioned, many of the prior known rail fasteners of the shear pad type are provided with openings in the rail plate for receiving clamping elements, for example, therein. Usually these openings extend through the elastomeric layer to the bottom plate of the shear pad. These openings provide pockets for accumulating debris which may eventually form an electrical contact between the rail plate and the bottom plate of the shear pad. Since present day rail fasteners are required to provide electrical insulation between the rail and the support structure, such accumulation of debris can destroy the electrical insulation capability of a rail fastener.